Xenix

Xenix is a discontinued Unix operating system for various microcomputer platforms, licensed by Microsoft from AT&T Corporation. The first version was released in 1980, and Xenix would eventually become the most common Unix variant, measured according to the number of machines on which it was installed, in the mid-to-late 1980s. The Santa Cruz Operation (SCO) later acquired exclusive rights to the software, and eventually replaced it with SCO UNIX, later known as OpenServer, with the final Xenix version released in 1991. Provided by Wikipedia